1. Field
The present invention relates to a method of manufacturing an insulating sheet, a method of manufacturing a copper clad laminate, and a method of manufacturing a printed circuit board, as well as a printed circuit board manufactured using these methods.
2. Description of the Related Art
In step with the advanced level of development in high-tech industries, such as the electrical, electronic, and aerospace industries, the printed circuit board (PCB) used in electronic equipment is expected to provide high levels of performance. In order to provide a lighter weight, lower thickness, and smaller size, the printed circuit board is being provided with thinner substrates, higher-density circuits, and finer patterns, etc., especially in a package for a semiconductor memory or computation device. Also, passive components, such as capacitors, etc., and active components, such as IC chips, may be embedded in the substrate, while the package itself may have a 3-dimensional form, with chips arranged in a stack.
In a printed circuit board thus provided with a higher density, as well as lighter weight, lower thickness, and smaller size, however, defects present in existing printed circuit boards, such as bending deformations in the substrate and degraded solder joint reliability due to differences in coefficients of thermal expansion between the substrate and the mounted chips, etc., may be exacerbated.
In order to prevent interference between circuits even in high frequencies, a printed circuit board having a higher density, lighter weight, lower thickness, and smaller size may also be required to provide a low dielectric constant (Dk) and low dielectric loss (Df). To obtain such properties, improvements are needed in the materials used for the printed circuit board, such as the copper clad laminate (CCL) and the prepreg (PPG).
A multi-layer printed circuit board according to the related art may be made as a composite of thermosetting resin, glass fibers, and an inorganic filler. Epoxy resin and BT (bismaleimide triazine) resin may generally be used for the main resin.
The epoxy resin used in a printed circuit board may form a 3-dimensional structure, as the epoxy groups in the resin molecules react with hardening agents to form cross-links. The types of epoxy resin commonly used can be divided into the bisphenol types and novolac types according to the preparation method.
Due to the movement towards environment-friendly products in current electric and electronic equipment, there is a demand for lead-free and halogen-free solder. As such, epoxy having a high glass temperature (Tg) and epoxy that does not include a bromine based flame retardant have also been developed.
A filler such as silicon dioxide (SiO2) may be added to an epoxy resin to complement the properties of the epoxy, while other fillers such as aluminum trihydrate (ATH) that serve as flame retardants may also be added. When using SiO2 fillers, which generally have a relatively lower coefficient of thermal expansion (CTE), fillers having smaller sizes may be used, in order to increase the content and hence the effect of SiO2, and fillers may be selected which have a greater effect of lowering the CTE.
In the case of glass fiber fabric used for the reinforcement material, E-glass may generally be used, while fabric made from high-strength glass fibers such as S-glass, NE-glass, T-glass, D-glass, etc., are also being used, in order to provide lower CTE.
A glass fiber fabric may be impregnated with a resin to produce a B-stage prepreg, while several layers of prepreg and copper foils may be stacked, heated, and compressed to produce a copper clad laminate.
In addition to epoxy resin, BT resin may often be used for the main resin in a printed circuit board intended for use in a package, because it may provide more desirable characteristics than does epoxy resin in terms of thermal properties (e.g. high Tg), electrical properties, and copper foil peel strength, etc., and because it is structurally very stable.
The thermal properties can be especially important, since the board for a package requires high reliability. That is, the coefficient of thermal expansion (CTE) may be different below and above the glass transition temperature (Tg) of the resin, and uneven contraction in volume during the manufacturing process may cause brittleness and warpage in the package board.
Furthermore, when uneven thermal expansion and contraction are repeated during the manufacturing process at around the glass transition temperature (Tg), residual stresses may be generated which may cause potential defects in the final product, such as delamination and warpage.
Existing materials used in manufacturing a package that display such properties may include BT or epoxy resin with a glass fiber fabric (e.g. E-glass type fabric), which has a low coefficient of thermal expansion but a high dielectric constant of about 6.2, so that the materials may have a relatively high dielectric constant of 3.5 to 4, as well as high dielectric loss. Thus, these materials may be difficult to use in high frequency regions (e.g. in the GHz range), and also, the fibers may not be readily impregnated by the resin.
Polyimide (PI), which may be used in a flexible copper clad laminate (FCCL), which is a material utilized in forming flexible and rigid-flexible PCB's, may have a rate of moisture absorption, and hence may entail dimensional instability, as well as a high dielectric constant (Dk) and high loss (Df).